Although hyperthermia is generally ones first thought when discussing heat issues in
athletics, hypothermia is the other end of the spectrum of what I will call "heat illnesses".
The "heat" equation for an individual at any point in their activity is the sum of heat production
from metabolism plus/minus the heat gained/lost from the environment. Heat energy is
a byproduct of working muscles. Conversion of
ATP to ADP and then into energy delivered to the pedals is a relatively inefficient process with
~70% of stored energy ultimately released a byproduct - heat. (And during exercise
heat production is increased approximately 20 times over your basal level.)

Hyperthermia

When there is an imbalance and heat production > heat loss, hyperthermia results. Muscle cramps may be the
first sign of hyperthermia. Next in the early spectrum of heat exhaustion is feeling
hot, fatigued, and nauseated, but still sweating. And finally there is heat stroke when
you stop sweating, your core temperature begins to rise (often >104 F), and nervous system symptoms
(altered state of consciousness or confusion) appear. It is potentially life threatening and
heat stress with exercise is associated with an increase sudden death as late as the next day. (In
military recruits there is an increase in exercise related deaths the morning
after intense exercise, especially in hot and humid conditions).

Heat gain from your surroundings is dependent on the ambient temperature. As problems
during exercise are usually related to shedding excess heat produced, let's focus on the
ways your body can eliminate excess heat energy in order to keep your temperature in a normal range.
Heat "unloading" occurs via:

radiation - an inefficient method

convection - more efficient when the air is moving over your body as in cycling

conduction - the way we lose heat while swimming

sweating - by far the most effective way to shed heat

Sweating is the only effective way to cool your body once the ambient temperature tops 92 degrees
Fahrenheit. And as it requires evaporation, its effectiveness drops off
quickly at high humidities, especially above 90% humidity. You have anywhere from 2 to 4 million
sweat glands and can lose
up to 2 to 4 liters of fluid an hour. In fact Alberto Salazaar was estimated to have lost 1 pint every
11 minutes in the 1984 Olympics. The heat energy loss from sweating is in the neighborhood
of 580 Calories per hour.

Another potential risk associated with heat illness is hyponatremia.
When you are losing sodium in sweat (15 - 50, avg 20 meq per liter, depending on sweat rate and
acclimatization) and replacing it with pure water, your serum sodium concentration can
drop below normal (hyponatremia) with serious consequences. Recent studies have
shown that there are "salt losers", those with high concentrations of salt in their sweat
(those with salt crusted on their jerseys after an
event), who may need even more than the normal salt replacement. It has been speculated that
their tendency to salt loss may be the result of these
athletes carrying a single gene for cystic fibrosis (present in 5% of Caucasians).

How do you protect yourself from hyperthermia? There are really just a few
things you can do:

Here are a couple of riding tips if you are going to be in a competitive event.

Pace yourself. Remember that 3 of every 4 Calories you are metabolizing (and the faster you go,
the more Calories you will burn per minute) are converted to heat. Thus the more you
are using per minute, the more potential for the body's heat balance to shift towards hyperthermia. Go slower and you
will be able to maintain a pace longer in extremely hot conditions.

Time your ride, if possible. It is cooler in the early morning (and late in the evenings). So get on
the road early if you can.

Fluids - don't overcompensate. Over the last few years the thinking has shifted from "drink
before you are thirsty" towards allowing your thirst to dictate your fluid intake. But this means
you have to listen to your body. Ignore your thirst and the risks of dehydration go up quickly.

If you are going to be riding more than 4 hours, you should think about additional
salt in your food and fluids. Sports drinks are not enough. Salty nuts, salty foods, and even salt tablets
may be needed.

Hypothermia

Hypothermia is a decrease in your core body temperature. It is always a risk for the
cyclist, especially in that combination of conditions
that often occur with winter training.

What are the warning signs?

Mild hypothermia (core body temperature above 95 degrees) - Uncontrollable shaking.
Hands and feet turn white. Pulse may temporarily quicken. Decrease in manual dexterity -
those hands just don't want to make the shifts or engage the brakes. This is a
potentially life threatening situation, particularly if the conditions that led to
it are ignored. The best thing to do is stop, find some warm shelter, and not even
consider getting back on the bike until the shivering stops.